3T3-L1 cells were treated with methylisobutylxanthine, dexamethasome and insulin to induce fatty metamorphosis. Incubation of differentiated 3T3-L1 cells with 100 pM insulin, 10 muM epinephrine, 0.1 mM adenosine, 15 ng/ml TSH, or 2 muU/ml ACTH rapidly increases the activity of a particulate cAMP phosphodiesterase. Incubation of differentiated cells with dexamethasone (10 nM for 72 hr) blocks the effect of insulin but not of epinephrine; higher concentrations of dexamethasone (at least 100 nM for 72 hr) are required to inhibit the effect of epinephrine.